Specifications

16
COMPUTER CONTROL
Serial Interface Electrical Connections & Cabling:
The 9-pin Subminiature D (male) connector on the SPM522D rear panel provides
the RS-232 compatible serial interface signals used for computer control. The SPM522D transmits serial data on pin 3 (TxD) and receives
serial data on pin 2 (RxD). The serial interface ground is on Pin 5. The DTR & RTS signals are connected to the +12 Volt power supply
(each through its own resistor) and are always asserted when the SPM522D power is on. Most IBM compatible PCs use either 25-pin or 9-
pin (male) connectors for their serial ports. The following table summarizes the pin assignments for the SPM522D serial interface, and for
the standard IBM compatible 9-pin and 25-pin serial ports.
SIGNAL NAME DIRECTION SPM522D 9-PIN IBM-PC 9-PIN IBM-PC 25-PIN
CD (carrier detect) input n/a pin 1 pin 8
RxD (receive data) input pin 2 pin 2 pin 3
TxD (transmit data) output pin 3 pin 3 pin 2
DTR (data terminal ready) output pin 4 pin 4 pin 20
signal ground n/a pin 5 pin 5 pin 7
DSR (data set ready) input n/a pin 6 pin 6
RTS (request to send) output pin 7 pin 7 pin 4
CTS (clear to send) input n/a pin 8 pin 5
RI (ring indicator) input n/a pin 9 pin 22
The SPM522D only requires receive data (pin 2), transmit data (pin 3), and signal ground (pin 5) to be connected for successful data
communications. However, the PC may require that signals be present on the data set ready, clear to send, or carrier detect inputs, as
well as the receive data, transmit data, and signal ground pins. The diagrams on the following page show cables for interfacing to a PC
with either a 9-pin or a 25-pin serial port connector. In most cases, one or the other of these cables will work. However, success or failure
depends entirely on the actual computer hardware and software being used. When trying to solve an interfacing problem, the most
important thing to remember is that an output of one device should connect to one or more inputs of the other device, and that two outputs
should never be connected together. Also, keep in mind that the RS-232 specification calls for the cable length to be no greater than 50
feet (although it is not unusual to be able to operate over distances of 150 to 250 feet), and the connectors must be of the appropriate
gender (male or female) to mate properly. For best results, a shielded cable should be used, with the shield connected to signal ground.
Since the SPM522D serial interface ground is also tied (indirectly) to the analog signal ground, undesirable ground loops may occur when
the SPM522D is connected to a PC (if the system grounding is not carefully designed). For best performance, the PC ground and the
chassis ground of the SPM522D should be at the same potential, and the PC should get AC power from the same source as the SPM522D
(and any other audio equipment which is connected to the SPM522D).
Serial Interface Data Communications Parameters:
The SPM522D communicates through the serial interface at a rate of 9600 bits per
second, with 8 data bits, 1 stop bit, and no parity. The SPM522D utilizes a subset of the standard 7-bit ASCII character set. The eighth
data bit of each character (the most significant bit) should always be 0. The computer should not echo the characters it receives. The
computer should not be set for either hardware (DTR) or software (XON/XOFF) flow control. The baud rate may be changed to 2400 bits
per second by means of an internal DIP switch labelled ‘Opt. W’ (for access instructions and switch diagram see Options on pg. 6). The
‘Opt. W’ DIP switch is located on the right-center of the lower circuit board, adjacent to the microprocessor (U2). To select 2400 baud rate:
1) Disconnect power from the SPM522D. 2) Remove top panel. 3) Move ‘Opt. W’ DIP switch towards the rear panel, to the ‘off’ position.